Image signal processing unit and method

ABSTRACT

A pixel signal processing unit and a pixel signal processing method are provided, which are capable of obtaining color level signals for pixel signals which differ in sensitivity between colors, the color level signals making full use of the dynamic range. For processing the pixel signals provided by the color pixels of an image device, there are provided a first amplifier, a second amplifier and a computing unit. The first amplifier sets a first amplification factor for the pixel signals for each color pixel to adjust the sensitivities of the color pixels. The second amplifier amplifies the pixel signals, whose sensitivities have been adjusted, with a second amplification factor. The second amplification factor allows the color level signal having the maximum signal intensity in the pixel signals which have been adjusted in sensitivity and output from the first amplifier to have a signal intensity equivalent to the dynamic range of image processing performed in a latter stage. The computing unit subtracts a black level signal component from each of the pixel signals amplified by the second amplifier. After adjusting the sensitivities which differ between colors, the color level signals can be amplified up to the dynamic range of the image processing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromeach of the prior Japanese Patent Application No. 2005-157380 filed onMay 30, 2005, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image signal processing unit andmore particularly to amplification of the signal level of an effectivepixel region.

2. Description of Related Art

Japanese Published Unexamined Patent Application No. 2001-28716discloses a technique including a pixel signal gain amplification (P×GA)circuit. When amplifying pixel signals output from a CCD, pixel signalshaving variations in amplitude are input to a correlated double sampling(CDS) circuit and then transferred from the CDS circuit to the P×GAcircuit. In the P×GA circuit, the pixel signals are individuallyamplified according to colors with a variably-settable gain to uniformthe levels of the pixel signals. These signals are further amplified bya variable gain amplifier, so that amplified pixel signals includingamplified noise are obtained.

SUMMARY OF THE INVENTION

Herein, the pixel signals output from the pixels which constitute animage device such as CCD through a CDS circuit are signals having acolor level signal on which a black level signal is superimposed. Theblack level signal is a signal output even in a light-shielded conditionand mainly caused by dark current flowing as leak current. The leakcurrent such as dark current is generated depending on the temperatureof the image device, device configuration, manufacturing variations,etc. and flows irrespective of pixels. Therefore, the pixel signalsoutput from the CDS circuit include the black level signal that is anoise component and superimposed on the color level signal indicative ofeffective picture information.

In the patent publication No. 2001-28716, each of the pixel signalsoutput from the CDS circuit is amplified in the P×GA circuit and thevariable gain amplifier and at that time, the color level signal and theblack level signal are simultaneously amplified. Therefore, even thoughthe amplified pixel signal seemingly has a signal level whichcorresponds to the limit of the dynamic range of an AD converter locatedin a latter stage, the black level signal, that is, an amplified noisecomponent is contained in this pixel signal. Since the color levelsignal which is effective as picture information cannot be amplified tothe limit of the dynamic range of the AD converter, there arises apossibility that the resolution of the picture information to betransmitted may decrease. This causes such a problem that the pictureinformation provided by the image device deteriorates.

Although the pixel signal is seemingly amplified up to the dynamicrange, the black level signal i.e., a noise component is superimposed onthe color level signal so that the resolution of the effective pictureinformation is likely to deteriorate.

The invention has been made taking account of the above-describedbackground art and therefore a primary object of the invention is toprovide an image signal processing unit and an image signal processingmethod which can perform image processing with color level signals whichmake full use of the dynamic range. In the invention, pixel signals eachincluding a black level signal superimposed on a color level signalwhich varies in sensitivity between colors are processed such that afterthe sensitivity of each color level signal is adjusted, the color levelsignal is amplified to the limit of the dynamic range of imageprocessing and then, the black level signal is eliminated from eachamplified pixel signal.

To achieve the purpose above, an image signal processing unit forprocessing pixel signals is provided by color pixels of an image device,each pixel signal including a color level signal and a black levelsignal superimposed on the color level signal, the processing unitcomprising a first amplifier for setting a first amplification factorfor the pixel signals for each color pixel to adjust the sensitivitiesof the color pixels, a second amplifier for amplifying the pixelsignals, whose sensitivities have been adjusted, with a secondamplification factor which allows the color level signal having themaximum signal intensity in the pixel signals which have been adjustedin sensitivity and output from the first amplifier to have a signalintensity equivalent to a dynamic range of image processing performed ina latter stage, and a computing unit for subtracting a black levelsignal component from each of the pixel signals amplified by the secondamplifier.

In the image signal processing unit according to the invention, whenprocessing pixel signals each having a color level signal on which ablack level signal is superimposed, the first amplifier sets a firstamplification factor for each color pixel to adjust the sensitivities ofthe color pixels. The second amplifier amplifies each of the pixelsignals, whose sensitivities have been adjusted, with a secondamplification factor. In the computing unit, a black level signalcomponent is subtracted from each pixel signal which has been amplifiedby the second amplifier. By virtue of the process in which the pixelsignals adjusted in sensitivity are amplified with their respectivesecond amplification factors so that the maximum signal intensity of thecolor level signals becomes the signal intensity equivalent to thedynamic range, and then, the black level signal component is subtractedfrom each pixel signal, it becomes possible to extract color levelsignals which make full use of the dynamic range. In this way, the colorlevel signal indicative of effective picture information obtained fromeach color pixel is amplified up to the dynamic range of imageprocessing performed in the latter stage.

Furthermore, an image signal processing method for processing pixelsignals is provided by color pixels of an image device, each pixelsignal composed of a color level signal and a black level signalsuperimposed on the color level signal, the method comprising the stepsof adjusting the sensitivities between the color pixels by individuallyamplifying the pixel signal of each color pixel, amplifying the pixelsignals whose sensitivities have been adjusted in the sensitivityadjusting step, so that the color level signals of the pixel signals areincreased up to a dynamic range of image processing performed in alatter stage, and subtracting a black level signal component from eachof the pixel signals obtained in the step of amplifying the pixelsignals adjusted in sensitivity.

In the image signal processing method according to the invention, whenprocessing pixel signals having a color level signal on which a blacklevel signal is superimposed, the pixel signals of the color pixels areindividually amplified to adjust the sensitivities differing between thecolor pixels and then, the pixel signals, whose sensitivities have beenadjusted, are amplified so that the color level signals expand up to thedynamic range of image processing performed in the latter stage,followed by subtraction of the black level signal component from eachpixel signal. Thereby, color level signals having signal levels whichmake full use of the dynamic range can be extracted.

The above and further objects and novel features of the invention willmore fully appear from the following detailed description when the sameis read in connection with the accompanying drawings. It is to beexpressly understood, however, that the drawings are for the purpose ofillustration only and are not intended as a definition of the limits ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram showing a pixel array of an image device;

FIG. 2 is a diagram showing an output signal of the image device;

FIG. 3 is a circuit block diagram of an image signal processing unitaccording to an embodiment;

FIG. 4 is a conceptual diagram showing a sensitivity adjustment made bya first amplifier (CDSamp);

FIG. 5 is a conceptual diagram showing processing performed by a secondamplifier (PGA) and a computing unit; and

FIG. 6 is a flow chart of an image signal processing method according tothe embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 to 6, an image signal processing unit and animage signal processing method will be described in detail according toa preferred embodiment of the invention.

The invention is applied to cases where image processing is performedwhile repeatedly obtaining picture information, when displaying movingimages or still images. In the following description, a digital camerawill be explained as an example of image processing units.

FIG. 1 shows a conceptual diagram of one example of the pixel array of asingle-panel image device 1 such as a CCD device for use in a digitalcamera etc. The image device 1 has pixels of three primary colors R, G,B, i.e., a red pixel R, a green pixel G and a blue pixel B to obtainpicture information.

The image device 1 has an effective image region 2 in which the colorpixels R, G, B for providing effective picture information are arranged.In the effective image region 2, rows in which the red and green pixelsR, G are alternately arranged in a lateral direction of FIG. 1 alternatewith rows in which the green and blue pixels G, B are alternatelyarranged in a lateral direction of FIG. 1, such that the green pixels Gin the adjacent rows are not aligned in a vertical direction. This arrayis the so-called “bayer array”.

Generally, the image device 1 has, in the periphery of the effectivepixel region 2, a light-shielded region 3 which is shielded from lightby a film made from e.g., argentine (Ag) or aluminum (Al). Thelight-shielded region 3 is called “an OB region” and also called “ablack level region” because it does not allow light transmission anddoes not acquire picture information. Although the light-shielded region(OB region) 3 is shielded from light and generate no color level signalsbased on picture information, it generates black level signals which arenoise components and mainly caused by dark current flowing as leakcurrent. The leak current such as dark current is generated depending onthe temperature of the image device 1, the configuration of the imagedevice 1, manufacturing variations etc. The leak current flows in allpixels irrespective of the light-shielded region (OB region) 3 or theeffective pixel region 2.

The image device 1 is laterally scanned. FIG. 2 shows a waveform of anoutput signal CCDIN outputted from the image device 1 during the lateralscanning of the image device 1, the waveform being associated with a rowin which the red and green pixels R, G are alternately arranged. Thepixels respectively generate pixel signals with a precharging period(Pre) interposed. In the first two cycles, black level signals OBR, OBGare outputted from the OB region. In the subsequent cycles, the red andgreen pixels R, G are alternately output in a sequential manner. Eachcolor pixel R, G outputs a pixel signal in which a black level signalOBR or OBG (a noise component) is superimposed on a color level signalCR or CG. The signal intensities of the color level signals CR, CGdiffer between the color pixels R, G depending on effective pictureinformation, whereas the signal intensities of the black level signalsOBR, OBG do not differ between the color pixels R, G. To obtain accuratepicture information, it is necessary to eliminate the black level signalOBR, OBG or OBB from the pixel signal generated by each color pixel R, Gor B on condition that the color level signal CR, CG or CB is notaffected.

Insignificantly, though, the signal intensities of the black levelsignals OBR, OBG, OBB of the color pixels R, G, B may differ from eachother for the reason that the black level signals are mainly originatedfrom leak current such as dark current which may be dependent upondevice properties. Therefore, when eliminating the black level signalsOBR, OBG, OBB from their associated pixel signals, it is preferable toestimate the black level signal for each of the color pixels R, G, B.The pixels are arranged in the light-shielded region (OB region) 3 inaccordance with the arrangement of the pixels in the rows of theeffective pixel region 2, thereby obtaining the black level signals OBR,OBG, OBB of the color pixels R, G, B, respectively.

In the light-shielded region (OB region) 3, since the color pixels arearranged, in relation to the adjacent rows, at the upper and lower endsof the image device 1 which are out of the scanned area, the scannedarea may be extended in order to obtain the black level signals locatedin these parts.

In FIG. 3 which shows the circuit block diagram of the embodiment, thereare provided an image signal processing unit 10 and a DSP unit 20. Theoutput signal CCDIN from the image device 1 (FIG. 1) is input to theimage signal processing unit 10. The output signal CCDIN is an analogsignal, and analog signal processing such as signal amplification isperformed in the image signal processing unit 10. The signal which hasundergone the analog signal processing is A/D-converted to be output asa digital signal to the DSP unit 20. In the DSP unit 20, imageprocessing such as automatic adjustments of focus, exposure, etc. isperformed. The image-processed pixel signal returns to the image signalprocessing unit 10 after being subjected to arithmetic processing ifrequired. Thereby, appropriate analog processing conditions for the nextoutput signal CCDIN from the image device 1 are set in the image signalprocessing unit 10.

In the image signal processing unit 10, a CDS circuit 11 to which theoutput signal CCDIN is input is connected to a first amplifier (CDSamp)12 which is in turn connected to a second amplifier (PGA) 13. An addingterminal (+) of a computing unit 14 is connected to the second amplifier(PGA) 13 and its subtracting terminal (−) is connected to a D/Aconverter (DAC) 16. The output of the computing unit 14 is connected toan A/D converter (ADC) 15 and an A/D-converted color level signal issent from the A/D converter (ADC) 15 to the DSP unit 20. Stored in aregister unit 17 are various parameters which are calculated by the DSPunit 20 and required for the analog signal processing performed by theimage signal processing unit 10.

The parameters stored in registers 17A to 17C are input to the firstamplifier (CDSamp) 12. A first amplification factor G1 is stored whichis used by the first amplifier (CDSamp) 12 for adjusting thesensitivities which differ between the color pixels R, G, B. Theparameter stored in a register 17D is input to the second amplifier(PGA) 13. A second amplification factor G2 is stored which is used forexpanding the color level signals effective as picture information up tothe dynamic range of the A/D converter (ADC) 15. The parameters storedin registers 17E to 17G are input to the D/A converter (DAC) 16. In thecomputing unit 14, the black level signals OB of the light-shieldedregion (OB region) 3 for the color pixels are stored in order torespectively eliminate the black level signals OB from their associatedpixel signals amplified by the first and second amplifiers 12, 13.

A timing signal CLK is input to the first amplifier (CDSamp) 12, the A/Dconverter (ADC) 15 and the D/A converter (DAC) 16. The timing signal CLKis a signal outputted whenever the pixel scanned is changed from one toanother during scanning of the image device 1. An OB clamp signal CLPOBis input by negative logic to an enable terminal (EN) of the computingunit 14. The OB clamp signal CLPOB is a signal activated while the lightshielded region (OB region) 3 of the image device 1 is scanned. Herein,the high-level state is regarded as an activated state.

Next, there will be explained the operation of the image signalprocessing unit 10 having the configuration described earlier. Theoutput signal CCDIN from the image device 1 includes two types ofsignals. The CDS circuit 11 samples the output signal CCDIN with thetiming of outputting each signal, thereby obtaining these signals. Oneof these signals is a reset level signal which is a reference signalsampled during a precharging period (Pre) (see FIG. 2). The other signalis a signal sampled in a pixel signal read-out period subsequent to theprecharging period (Pre). A difference signal obtained by subtractingthis signal from the reset level signal is the pixel signal. In the CDScircuit 11, the output signal CCDIN is sampled based on a timing signal(not shown) during the precharging period (Pre) and the pixel signalread-out period subsequent to it and the pixel signal, that is, thedifference signal is released.

The pixel signals output from the CDS circuit 11 are signals in whichthe black level signals OBR, OBG or OBB are superimposed on the colorlevel signal CR, CG or CB indicative of effective picture information.Generally, the sensitivities of the pixel signals differ between thecolor pixels R. G, B. The sensitivities of the red pixel R and bluepixel B are usually lower than that of the green pixel G. In the firstamplifier (CDSamp) 12, a first amplification factor G1 is set for eachof the color pixels R, G, B and the pixel signals are individuallyamplified with their respective amplification factors G1 in order toadjust the difference between the sensitivities of the color pixels R,G, B.

The process of this adjustment is shown in FIG. 4. FIG. 4 shows a casewhere the signal intensities of the pixel signals CR (AV)+OBR (AV), CG(AV)+OBG (AV), CB (AV)+OBB (AV) generated from the color pixels R, G, Bwhich are diagrammatically shown as the output signals of the CDScircuit 11 are such average signal intensities that the signalintensities of the color level signals CR (AV), CG (AV), CB (AV)correspond to the sensitivities of the color pixels R, G, B.

To eliminate the difference in sensitivity between the color levelsignals CR (AV), CG (AV), CB (AV), the signal intensities of the colorlevel signals CR (AV), CG (AV), CB (AV) are adjusted to the maximumsignal intensity possessed by the color level signal CG (AV). Morespecifically, first amplification factors G1R, G1G, G1B are set for thecolor level signals CR (AV), CG (AV), CB (AV) respectively, theseamplification factors having been calculated by the DSP unit 20 andstored in the registers 17A to 17C.

Herein, the amplification factor G1R for the red pixel R is calculatedfrom G1R=CG(AV)/CR(AV). The amplification factor G1B for the blue pixelB is calculated from G1B=CG (AV)/CB(AV). The amplification factor G1Gfor the green pixel G is described by G1G=CG(AV)/CG(AV)=1. As describedlater with reference to FIG. 6, these amplification factors can becalculated beforehand through digital arithmetic processing performed bythe DSP unit 20.

The registers 17A to 17C are sequentially selected in response to thetiming signal CLK. Alternatively, if the first amplification factorsG1R, GIG, G1B stored in the registers 17A to 17C have already been takenin the first amplifier (CDSamp) 12, the first amplification factors G1R,G1G, G1B are sequentially selected in the first amplifier (CDSamp) 12.Thereby, individual amplification is performed for every color pixel R,G, B and the first amplifier (CDSamp) 12 outputs signals obtained byadjusting the sensitivity deviation of each of the color pixels R, G, B.The color level signals CR (AV), CG (AV), CB (AV), which have averagesignal intensities corresponding to the sensitivities of the colorpixels R, G, B respectively, are amplified. Concretely, the color levelsignal CR (AV) is amplified by the first amplification factor G1R andthe color level signal CB (AV) is amplified by the first amplificationfactor G1B so that their signal intensities become equivalent to that ofthe color level signal CG (AV) It should be noted that the black levelsignals OBR (AV), OBG (AV), OBB (AV) are similarly amplified at the sametime.

The pixel signals adjusted insensitivity are output from the firstamplifier (CDSamp) 12 and input to the second amplifier (PGA) 13. In thesecond amplifier (PGA) 13, amplification is performed by use of thesecond amplification factor G2.

Herein, each of the pixel signals to be amplified by the first amplifier(CDSamp) 12 has a distribution of various signal intensitiescorresponding to picture information obtained from the effective pixelregion 2 of the image device 1. FIG. 5 diagrammatically shows pixelsignals which are obtained by amplifying such pixel signals of the colorpixels R, G, B with the first amplification factors G1R, G1G, G1Brespectively so that their sensitivities are adjusted. If a color levelsignal CMAX having the maximum signal intensity coincides with thedynamic range DR of the A/D converter (ADC) 15 located in the latterstage, all of the pixel signals adjusted in sensitivity will beeffectively A/D-converted, making best use of the conversion ability ofthe A/D converter (ADC) 15.

In the second amplifier (PGA) 13, arithmetic processing is performed bythe DSP unit 20 beforehand and the second amplification factor G2 storedin the register 17D is set as the amplification factor to be used. Thesecond amplification factor G2 is calculated as the ratio of the dynamicrange DR of the A/D converter (ADC) 15 to the color level signal CMAXhaving the maximum signal intensity, based on the color level signalsobtained in the preceding image processing. In this case, the blacklevel signals OB are amplified with the second amplification factor G2at the same time.

Each pixel signal amplified by the second amplifier (PGA) 13 is such asignal that the color level signal component is amplified up to thedynamic range DR by the second amplification factor G2 and the blacklevel signal component, which is also amplified with the secondamplification factor G2, is superimposed on the color level signal.Therefore, the black level signal has to be subtracted from each pixelsignal in order to A/D-convert the color level signal by the A/Dconverter (ADC) 15, making full use of the dynamic range. Thissubtraction is performed by the computing unit 14.

The black level signal to be subtracted in the computing unit 14 is theblack level signals OBR, OBG and OBB which are respectively superimposedon the pixel signals output from the pixel device 1 and amplified by thefirst and second amplifiers 12, 13. Hereinafter, this black level signalto be subtracted is referred to as “the amplified black level signal”.As described later in conjunction with FIG. 6, the amplified black levelsignal OB (AV) is obtained through the processing performed by the DSPunit 20 in which the black level signals OBR, OBG or OBB obtained fromthe light-shielded region (OB region) 3 are integrated and averaged asthe black level signal which corresponds to the pixel signal of eachcolor pixel R, G or B, by relating the pixels of the light-shield region(OB region) 3 to the pixel array of the effective pixel region 2. Thearithmetic processing is performed beforehand and the amplified blacklevel signals for the color pixels R, G, B are stored in the registers17E to 17G respectively.

In response to the timing signal CLK, the registers 17E to 17G aresequentially selected so that each amplified black level signal OB (AV)is D/A-converted by the D/A converter (DAC) 16. The amplified blacklevel signal OB (AV), which has been converted into an analog signal, isinput to the computing unit 14 and subtracted from the associated pixelsignal output from the second amplifier (PGA) 13, so that the colorlevel signal expanded up to the dynamic range of the A/D converter (ADC)15 is extracted and input to the A/D converter (ADC) 15.

FIG. 6 shows a flow chart of the pixel signal processing method. Afterstarting the processing, initialization is first performed (S1). Theinitialization defined herein includes: sensitivity adjustment for eachcolor pixel R, G, B; amplification up to the dynamic range intended forthe image processing performed in the latter stage; and setting of ablack level component to be subtracted from each pixel signal. Forinstance, the initialization is performed such that the sensitivityadjustment and the amplification up to the dynamic range are not done(in this case, the amplification factors are 1) and the black levelcomponent to be subtracted from each pixel signal is made to be zero.More specifically, in the image signal processing unit 10 shown in FIG.3, the initialization is performed such that all of the amplificationfactors G1, G2 stored in the registers 17A to 17D become 1 and the blacklevel signal components stored in the registers 17E to 17G are made tobe zero.

After picture information is fetched from the image device, automaticfocusing (S2), automatic exposure (S3), etc. are carried out as theimage processing and the pixel signals of one screen constituted by theeffective pixel region 2 and the light-shielded region (OB region ) 3are obtained (S4).

Of the pixel signals which have been obtained, the black level signalsOBR, OBG, OBB obtained from the light-shielded region (OB region) 3 areallocated to the color pixels R, G, B respectively in the order in whichthe color pixels are arranged in the rows of the effective pixel region2 and then averaged. Thus, an average black level signal OB (AV) foreach of the color pixels R, G, B is obtained (S5). The average blacklevel signals OB (AV) are stored in an adjusted black level signalstorage unit D1. More concretely, in the image signal processing unit 10shown in FIG. 3, the average black level signals OB (AV) are stored inthe registers 17E to 17G, respectively.

Then, the color level signals CR, CG, CB obtained by the imageprocessing are averaged, thereby obtaining a color level signal C (AV)for each of the color pixels R, G, B (S6). A color level signal C (AV)MAX having the maximum signal intensity is extracted from the averagecolor level signals C (AV) (S7). Then, the ratio of the maximum signalintensity to the signal intensity of the average color level signal C(AV) for each color pixel R, G, B is calculated (S8). The result of thearithmetic processing is the first amplification factorG1(=C(AV)MAX/C(AV)). The amplification factor G1 is calculated for everypixel R, G, B and stored in the first amplification factor (G1) storageunit D2. Specifically, in the image signal processing unit 10 shown inFIG. 3, the amplification factors G1 are stored in the registers 17A to17C, respectively. It should be noted that Steps S7 to S8 correspond tothe processing shown in FIG. 4.

The maximum signal intensity CMAX is extracted from the color levelsignals C (S9) and the ratio of the signal intensity DR corresponding tothe dynamic range of the image processing to the maximum signalintensity CMAX is calculated (S10). The result of the arithmeticprocessing is the second amplification factor G2(=DR/CMAX) which is tobe stored in a second amplification factor (G2) storage unit D3.Specifically, in the image signal processing unit 10 shown in FIG. 3,the second amplification factor G2 is stored in the register 17D. StepsS9 to S10 correspond to the processing shown in FIG. 5.

After the above step, the program returns to Step S2 to repeat the imageprocessing. For the pixel signal to be subjected to the imageprocessing, the black level signal OB (AV), the first amplificationfactor G1 and the second amplification factor G2, which have beenobtained from the calculation in the preceding image processing, areutilized. Thereby, a sensitivity adjustment is made to each color pixelR, G, B and the signal intensity of each color level signal indicativeof picture information is increased up to the dynamic range so that theimage processing can be performed, making full use of the dynamic range.

The flow chart of FIG. 6 shows a flow of the image processing executed,for example, by the DSP unit 20 in the circuit block diagram of theembodiment shown in FIG. 3.

As fully discussed earlier, according to the image signal processingunit and image signal processing method of the present embodiment, thesignal level can be increased up to the dynamic range DR of the imageprocessing performed by the A/D converter (ADC) 15 and others, byamplifying the color level signals CR, CG, CB obtained by eliminatingthe black level signals OBR, OBG, OBB from the pixel signals. Theconventional technique, in which signal intensity is increased bydigital signal processing after A/D conversion, has not provedsuccessful in fully utilizing the resolution of the A/D converter,because the signal intensity of the color level signals converted by theA/D converter does not reach the dynamic range of the A/D converter. Incontrast with this, the present embodiment makes amplification that isanalog signal processing, whereby the color level signal C can beincreased up to a signal level which is high enough to make full use ofthe dynamic range of the A/D converter.

Prior to the conversion by the A/D converter (ADC) 15 and other imageprocessing processes, the black level signals OBR, OBG, OBB, which arethe noise components of the pixel signals, are eliminated at the stageof the analog signal processing. Only the color level signals CR, CG,CB, which are effective picture information, are amplified to a maximumextent and sent to the A/D converter (ADC) 15 and to other imageprocessing devices. Thus, the picture information provided by the imagedevice 1 can be processed by making full use of the dynamic range DR andthe image processing by the DSP unit 20 and other image processingprocesses can be performed with the maximum resolution. As a result, theability of the image device 1 can be utilized to the fullest possibleextent.

The black level signals OBR, OBG, OBB to be eliminated from the pixelsignals are calculated, using the pixel signals obtained from thelight-shielded region (OB region) 3 of the image device 1. The pixelarrangement of the light-shielded region (OB region) 3 has the samecontinuity as that of the pixel array of the effective pixel region 2.Therefore, the black level signal OBR (OBG, OBB) corresponding to eachof the color pixels R, G, B can be obtained by averaging the black levelsignals OBR (OBG, OBB) located at the pixel positions corresponding tothe pixel array of the effective pixel region 2.

In the sensitivity adjustment made to every pixel R (G, B), the colorlevel signal CR (CG, CB) may be taken out from the pixel signal andcompared to the average signal intensity of the entire effective pixelregion 2 to make an adjustment. This leads to an improvement in theaccuracy of the sensitivity adjustment, compared to the techniquedisclosed in Japanese Published Unexamined Patent Application No.2001-28716 according to which the SN ratio of each pixel signal isimproved while the pixel signal contain the black level signal so thatthe black level signal, i.e., a noise component is inevitably amplified,resulting in poor accuracy in the sensitivity adjustment made to each ofthe color pixels R, G, B.

It is apparent that the invention is not necessarily limited to theparticular embodiment shown herein and various changes and modificationsare made to the disclosed embodiment without departing from the spiritand scope of the invention.

While the image device has been discussed taking a single-panel CCD forexample in the foregoing embodiment, the invention is not limited tothis. The invention is equally applicable to a so-called three-paneldevice configuration in which an image device is provided for each ofthe color pixels R, G, B.

According to the invention, the color level signal which is obtained byeliminating the black level signal from the pixel signal can beamplified up to the dynamic range. Since the black level signal, i.e.,the noise component of the pixel signal is eliminated, the color levelsignal which provides effective picture information can be amplified upto a maximum extent. The picture information provided by the imagedevice can be transmitted with the resolution corresponding to the upperlimit of the dynamic range, so that the image processing at the laterstage can be performed with the maximum resolution. In consequence, theability of the image device can be utilized to the fullest possibleextent.

1. An image signal processing unit for processing pixel signals providedby color pixels of an image device, each pixel signal including a colorlevel signal and a black level signal superimposed on the color levelsignal, the processing unit comprising: a first amplifier for setting afirst amplification factor for the pixel signals for each color pixel toadjust the sensitivities of the color pixels; a second amplifier foramplifying the pixel signals, whose sensitivities have been adjusted,with a second amplification factor which allows the color level signalhaving the maximum signal intensity in the pixel signals which have beenadjusted in sensitivity and output from the first amplifier to have asignal intensity equivalent to a dynamic range of image processingperformed in a latter stage; and a computing unit for subtracting ablack level signal component from each of the pixel signals amplified bythe second amplifier.
 2. The image signal processing unit according toclaim 1 further comprising an A/D converter for A/D-converting outputsignals of the computing unit, wherein the dynamic range of imageprocessing performed in the latter stage is the dynamic range of the A/Dconverter.
 3. The image signal processing unit according to claim 1,wherein the first amplification factor, the second amplification factorand the black level signal component are determined based on a result ofpreceding image processing previously performed.
 4. The image signalprocessing unit according to claim 3, wherein the image processing isdigital arithmetic processing, and wherein the first amplifier sets, inthe preceding image processing, the first amplification factor for eachcolor pixel according to the ratio of a maximum signal intensity to asignal intensity of each color pixel, the maximum signal intensity beingthe highest in the signal intensities of the average color level signalsof the color pixels, the signal intensity of each color pixel being thesignal intensity of the average color level signal of each color pixel.5. The image signal processing unit according to claim 3, wherein theimage processing is digital arithmetic processing, and wherein thesecond amplifier sets, in the preceding image processing, the secondamplification factor according to the ratio of a signal intensityequivalent to the dynamic range to the maximum signal intensity which isthe highest in the signal intensities of the color level signals.
 6. Theimage signal processing unit according to claim 3, wherein the imageprocessing is digital arithmetic processing, and wherein the computingunit performs, in the preceding image processing, subtraction operationaccording to an average black level signal obtained by averaging theblack level signals for each color pixel, the black level signals beingprovided by a light-shielded region of the image device.
 7. The imagesignal processing unit according to claim 6, further comprising a D/Aconverter for D/A-converting the average black level signal obtained bythe digital arithmetic processing, wherein the computing unit includesan analog subtracter for subtracting the D/A-converted black levelsignal from its associated pixel signal amplified by the secondamplifier.
 8. The image signal processing unit according to claim 6,wherein the light-shielded region is located in the periphery of aneffective pixel region, and wherein the black level signalscorresponding to the order of the light-shielded region are allocated tothe color pixels, according to the order of the color pixels arranged inthe effective pixel region.
 9. The image signal processing unitaccording to claim 1, wherein the black level signal componentsubtracted by the computing unit is obtained by amplifying the blacklevel signals by the first amplifier and the second amplifier.
 10. Animage signal processing method for processing pixel signals provided bycolor pixels of an image device, each pixel signal composed of a colorlevel signal and a black level signal superimposed on the color levelsignal, the method comprising the steps of: adjusting the sensitivitiesbetween the color pixels by individually amplifying the pixel signal ofeach color pixel; amplifying the pixel signals whose sensitivities havebeen adjusted in the sensitivity adjusting step, so that the color levelsignals of the pixel signals are increased up to a dynamic range ofimage processing performed in a latter stage; and subtracting a blacklevel signal component from each of the pixel signals obtained in thestep of amplifying the pixel signals adjusted in sensitivity.
 11. Theimage signal processing method according to claim 10, wherein thedynamic range of image processing performed in the latter stage is thedynamic range when the pixel signals from which the black level signalcomponent has been subtracted are A/D-converted.
 12. The image signalprocessing method according to claim 10, wherein the image processing isdigital arithmetic processing, the method further comprising the stepsof: averaging the signal intensities of the color level signals for eachcolor pixel; and calculating each ratio of a maximum signal intensity toa signal intensity of each color pixel as each first amplificationfactor for the pixel signals for each color pixel, the maximum signalintensity being the highest in the signal intensities of the averagecolor level signals of the color pixels, the signal intensity of eachcolor pixel being the signal intensity of the average color level signalof each color pixel, the averaging step and the calculating step beingperformed based on the result of preceding image processing previouslyperformed, prior to the sensitivity adjusting step.
 13. The image signalprocessing method according to claim 10, wherein the image processing isdigital arithmetic processing, the method further comprising the stepof: calculating, as a second amplification factor for the pixel signalsadjusted in sensitivity, the ratio of a signal intensity equivalent tothe dynamic range to the maximum signal intensity which is the highestin the signal intensities of the color level signals, based on theresult of preceding image processing previously performed, prior to thestep of amplifying the pixel signals adjusted in sensitivity.
 14. Theimage signal processing method according to claim 10, wherein the imageprocessing is digital arithmetic processing, the method furthercomprising the step of: calculating the black level signal component foreach color pixel by averaging the black level signals for each colorpixel which are provided by the light-shielded region of the imagedevice, based on the result of preceding image processing previouslyperformed, prior to the step of subtracting the black level signalcomponent.
 15. The image signal processing method according to claim 14,further comprising the step of D/A-converting the average black levelsignal, wherein the step of subtracting the black level component isanalog arithmetic processing.
 16. The image signal processing methodaccording to claim 14, wherein the black level signals corresponding tothe order of the light-shielded region are allocated to the colorpixels, according to the order of the color pixels arranged in aneffective pixel region.
 17. The image signal processing method accordingto claim 10, wherein the black level signal component subtracted fromeach pixel signal is obtained by the amplification of the black levelsignals in the sensitivity adjusting step and in the step of amplifyingthe pixel signals adjusted in sensitivity.